A Gage R&R is a scientific study that falls under the larger topic of Measurement System Analysis (MSA). When you observe any measurement, it will typically vary from time to time. That variation in the observed measurement will be the result of the variation in the object that you are measuring plus the variation of the measuring device.
For example, when you get on the bathroom scale each morning, the scale reading is usually different. That variation will be the variation in your weight plus the variation in the scale mechanism. The Gage R&R helps you distinguish how much of the variation is due to the scale mechanism.
If the proportion of scale mechanism variation is a large part of the overall variation, then you won’t really know your true weight — or whether your new diet is working. The Gage R&R study specifically measures the repeatability and reproducibility of the measurement system.
Repeatability is your ability to measure the same thing multiple times with the same measuring device, under the same circumstances and get the same answer. Reproducibility is the ability of you and your friend Charles to measure the same thing multiple times with the same measuring device, under the same circumstances and both of you getting the same answer.
The Gage R&R study is a carefully planned process by which you select a number of “operators,” or people to do the study, and a number of objects to measure that are known to have inherent variation. You will also choose a measuring device such as a ruler, timer, or scale. Usually you’ll select 2-3 people and 5-10 objects. Each person will measure each object multiple times, usually 2-3 times. You are not interested in the actual values but in the variation that exists when a person measures an object multiple times and the variation that will exist between the different people who are measuring the same object.
The purpose of the same person measuring each object multiple times is to assess the repeatability of the measurement system. Using multiple people to measure the same object multiple times will assess the reproducibility of the measurement system. Calculations of the variance are made of the overall variation of the process plus the variation due to the repeatability and reproducibility. The results are then presented as a ratio of the repeatability and reproducibility and the overall variation. This is expressed as a percentage contribution of the R&R as a function of the overall variation.
Generally recognized criteria for gage acceptability is when Gage R&R variability to overall process variability is:
If you want to make good data-driven decisions, you must have faith that your measurement system is providing good data. Here are a few of the benefits of doing a Gage R&R study.
While you want your measurement system to provide both accuracy and precision, the Gage R&R deals strictly with the precision component of measurement variation and provides a simple percentage contribution of measurement variation as a function of overall variation. You want as little contribution from measurement issues versus actual process variation as possible.